The present invention relates generally to particle deposition and, more particularly, to a system and methods for efficiently depositing small particles on substrates.
Particle scanners are used to detect contamination on the surfaces of semiconductor wafers, computer disks, flat panel display glass, and other industrial substrates that may be sensitive to contamination. In general, these scanners operate by sensing light scattered by particles as a laser is scanned over the substrate surface. To calibrate these scanners, particles of a known size and known diameter distribution are used. The particles typically used in scanner calibration are polystyrene latex (PSL) spheres. To create a sample for use in scanner calibration, a particle deposition system is used to deposit a known amount of PSL spheres having a known particle size and a known diameter distribution onto the surface of the sample. During the deposition process, it is important that the particle deposition system does not introduce other contamination onto the sample surface.
Known particle deposition systems typically atomize a suspension of particles, e.g., PSL spheres, in clean water (or other fluid) to form a spray in a flow of clean air. After the particles dry, they are either led directly to the sample or may pass through any of several devices to control or monitor flow rates and electrically charge or discharge the particles. One common technique is to pass the particle flow through a differential mobility analyzer (DMA), which passes only a selected band of charged particle diameters through a narrow slit in a rod. An electric field draws the charged particles sideways through the clean, laminar airflow toward the rod. The smaller particles are drawn through the airflow more easily and therefore reach the rod before larger particles. By adjusting the electric field, the particle diameter that passes the slit an be selected. Control of the particle diameter selection process depends on the temperature and flow parameters in the system as well as the voltage controlling the electric field.
It has recently become more important to control precisely the particle diameter selection process to satisfy the semiconductor industry""s need for more precise information regarding the true size of the particles being deposited. Commercially available particle deposition systems do not adequately monitor all of the parameters needed to obtain long-term control of the DMA. Consequently, these systems suffer from an unwanted variation in the relationship between the particle diameter passing the slit and the applied control voltage.
U.S. Pat. No. 5,534,309 to Liu discloses a method and apparatus for depositing particles on a semiconductor wafer. The disclosed method requires that a purging step be conducted to remove unwanted particles from the deposition chamber before the wafer is placed in the chamber. Another purging step is conducted at the end of each deposition before the wafer is removed from the chamber. These purging steps require that the wafer be removed from the deposition chamber at the end of each deposition. Thus, the production of sample wafers having multiple spot depositions thereon requires that the wafers must be moved into and out of the deposition chamber repeatedly. This frequent handling of the wafers significantly slows the production of sample wafers having multiple spot depositions. In addition, it also may introduce the wafers to contamination, which is undesirable in clean room operations.
In view of the foregoing, there is a need for a particle deposition system that enables the size of the particles being deposited to be precisely controlled and that reduces the time required to produce standards, e.g., sample wafers for use in scanner calibration.
Broadly speaking, the present invention fills this need by providing a particle deposition system that controls the flow of gas containing particles into a deposition chamber so that unwanted particles do not enter the deposition chamber. The particle deposition system also monitors the flow of the gas containing the particles so that the deposited particle diameter remains substantially constant during the deposition process.
In accordance with one aspect of the invention, a method for depositing particles onto a substrate is provided. In this method, a flow of gas containing particles is provided along a flow path that bypasses a deposition chamber. To deposit particles onto a substrate in the deposition chamber, the flow path of the gas containing the particles is changed so that the flow of the gas containing the particles causes particles to be deposited onto the substrate. After a desired amount of particles have been deposited onto the substrate, the flow path of the flow of the gas containing the particles is changed to the flow path that bypasses the deposition chamber.
In one embodiment, the flow of the gas containing the particles flows into a vacuum that is coupled in flow communication with the deposition chamber. In this embodiment, the vacuum is configured to prevent the flow of the gas containing the particles from entering the deposition chamber without drawing any significant amount of air from the deposition chamber. In one embodiment, the flow of the gas containing the particles is diverted from the vacuum to the deposition chamber by interrupting the flow communication between the vacuum and the flow of the gas containing the particles.
In another embodiment, particles are deposited onto the substrate in accordance with a first set of deposition parameters. Thereafter, without removing the substrate from the deposition chamber, particles are deposited onto the substrate using a second set of deposition parameters. In one embodiment, particles having the same particle size are deposited in different spot locations on the substrate. In another embodiment, particles having different particle sizes are deposited in different spot locations on the substrate.
In accordance with another aspect of the invention, a method for maintaining particle diameter during deposition of particles onto a substrate is provided. In this method, gases are flowed into a differential mobility analyzer (DMA) having a slit for passing particles therethrough. At least one of the gases flowed into the DMA contains particles. The gases flowing into and out of the DMA are monitored, and periodic adjustments are made to the voltage applied to the DMA so that a particle size diameter passed through the slit remains substantially constant during the deposition process. In one embodiment, the monitoring of the flows into and out of the DMA includes measuring the pressure differential across orifices located before and after the DMA.
In accordance with yet another aspect of the invention, a particle deposition system is provided. In one embodiment, the particle deposition system includes a deposition chamber having an inlet and a conduit coupled to this inlet. The conduit, which has a first branch and a second branch, is in flow communication with a source of gas containing particles. A particle counter is disposed in the first branch of the conduit and an orifice is disposed in the second branch of the conduit. A vacuum is coupled in flow communication with the first and second branches of the conduit.
In another embodiment, the particle deposition system includes an atomizer for providing a flow of gas containing particles. A flow control device is coupled in flow communication with the atomizer, and a differential mobility analyzer (DMA) is coupled in flow communication with the flow control device. A deposition chamber is coupled in flow communication with the flow control device and the DMA. When the particles in the flow of the gas containing the particles are to be filtered by the DMA, the flow control device directs the flow of the gas containing the particles toward the DMA. When the particles in the flow of the gas containing the particles are not to be filtered by the DMA, the flow control device directs the flow of the gas containing the particles toward the deposition chamber.
The present invention advantageously enables particles having a constant particle diameter to be deposited onto a substrate in a deposition chamber without introducing unwanted particles in the deposition chamber. This avoids the need to purge the deposition chamber between depositions. Consequently, multiple depositions can be made on a substrate without removing the substrate from the deposition chamber. By minimizing the handling of the substrate required for deposition, the present invention significantly decreases the time required to produce standards having multiple depositions thereon. It also minimizes the opportunity for contamination to be introduced onto the substrate during the deposition process.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.